This invention relates to the field of optical devices and, more particularly, to tunable optical filters.
In many applications of optical technology, it is necessary to isolate a particular wavelength or band of wavelengths from incident light containing a broad band of wavelengths. In the field of infrared detection, for example, a typical target object, such as an airborne vehicle, commonly will emit a characteristic infrared radiation spectrum containing features which are unique to that particular type of vehicle, such as one or more peak amplitudes at particular wavelenths in the spectrum. Consequently, it is essential to provide a detection apparatus with the capability of filtering incident light to obtain a signal with a narrow bandwidth centered about one or more specific wavelengths and with a high rejection for the off-band portions of the received light. Another typical application where such a filtering device is needed is in optical communications, where a carrier optical beam may be multiplexed with a plurality of different signals, each modulating a different frequency within the beam spectrum. A narrow band filter must be utilized to separate information contained on a particular wavelength from the remainder of the carrier beam.
One approach toward achieving the desired type optical filtering is to mechanically introduce a light filtering material into the path of an incoming optical beam, the characteristics of the particular material used then determining the wavelength or band of wavelenths which will be removed from the beam. Such mechaical systems, however, are undesirable since they are inherently slow in responding to changes in the transmitted signal. Thus, it is frequently desirable to utilize a system which may be adjusted, or turned, very rapidly. One class of devices which satisfies this requirement is optical filters whose transmission characteristics may be controlled by the application of an electric potential to the filter. Such filters have been used in a variety of applications, such as optical signal processing, high resolution spectroscopy, solar physics, astrophysics, high pressure tunable lasers, and infrared focal plane technology.
Unfortunately, however, a tradeoff commonly occurs in optical filter design between high resolution and large angular aperture capabilities, that is, when a filter is designed to provide a narrow pass band, that filter tends to function only for a relatively more narrow angle of incident light. Thus, a need has developed in the art for an electronically tunable optical filter exhibiting high resolution and operating over a wide angular aperture.